mxw1

Does anyone know what the EMC does with the inputs from the power steering switch, air conditioning and park/drive switch?

I'm assuming that it just steps the Idle Speed Control motor in anticipation of a load increase while the engine is idling.

I am using a 1988 TPI unit on a hot rod and never connected those inputs. I never had a very smooth idle and was just reading a troubleshooting tree that says to check these inputs if you have a poor quality idle.
I did have a 90 Corvette with TPI and that thing idled smoothly, so I'm disappointed with my hot rod's idle.

Maybe not having any input to these input wires allows spurious voltages on these ungrounded/ unconnected wires causing havoc with my idle???
Anyone have any experience with this?

Thanks,

Mike

aklim

I believe the AC on tells the IAC to increase the idle speed. I don't know about a Power Steering input. What troubleshooting tree did this come from? Something general or corvette specific?

What is your idle doing? Have you had the TB cleaned by taking the IAC, IAC housing, Top Plate off and cleaning the passages? Have you changed anything in the setup? Cam, intake, etc? Have you set it by IAC counts?

mxw1

The idle is rough. It sounds like the engine has a much wilder cam, but it is very mild.
The TPI is actually from a 1988 Camaro 305 with Mass Air Flow sensor. I put it in my hot rod about 10 years ago, it had a 350 SBC. I put 350 sized injectors when I installed it. I wore out the 350 and built a 406 SBC. I also replaced the intake with an LT-1 (1993 vintage)and 1990 Corvette aluminum heads, using the original distributor for 1988 TPI system. I believe I'm using Z06 injectors for the higher flow rates on the 406 engine.
It idles OK without stalling, but lopes. It's not so bad when cold, but when it goes into closed loop it's aggravating. I have a 700R4 automatic and I use a toggle switch for torque converter lock-up.
I'm not using the air pump, the diverter solenoids or EGR system. I have a vehicle speed sensor feeding the ECM.
I use this car as a commuter and drive 80 miles per day, mostly freeway driving.
Yes, this is a hodgepodge of parts, any of the above could be wrong, but it runs really well other than the idle, which seems to be getting worse. I've checked the ohms across each injector and all are at 12.5 ohms + or - 0.2 .
I replaced plugs, cap and rotor as well as the wires about a month ago, I haven't looked at my minimum idle or TPS voltage in a while, I should look there this weekend.
Yes, I know I have many modifications.

Mike

aklim

THAT right there is an issue. Bigger injectors and the ECM is set for a 305.

Another problem is that the ECM didn't know. I can put 42 pph injectors in it but if the injector constant isn't changed in the ECM, it assumes it is whatever is programmed into it and fires it like it should making adjustments where it can. When it goes out of the adjustment window, it results in a funky running engine.

Makes me think that there is definitely something wrong with the tables.

Shouldn't affect anything.

So YOU say. I disbelieve whatever the butt dyno says as a rule and work from what I know. Just because YOU think it is running good doesn't mean the ECM isn't quietly compensating. In a carbed car, the ECM cannot adjust and you have to guess what it needs.

Says nothing as to whether the injectors are the right sized ones or if they are even working. All you know is the coils seems to be right. I can cut off the bottom half and it will ohm out right. At best, it tells you the coil is bad and you replace the set. At worse, it gives you the false sense of security and you go look elsewhere for the answer. Again, I actively distrust the butt dyno and prefer to have it actually checked on a flow bench to see what it is doing.

At this point, I wouldn't bother.

You have many parts thrown at it and it has to work as a system. What you are doing is akin to throwing a bunch of guys together and expect them to play football and do it well. Not happening. You have to match their expertise to each other and form a real team to have a shot. Same thing. I'd find out what the injector size is first. After which, get the specs for the parts. After that, bring it to a dyno facility to get the ECM tuned. Then you can go adjusting whatever you need. I have no idea what your desired idle speed is so that needs to be set in the ECM. That will determine things

mxw1

I found the TPS (throttle position sensor) was at 800 or 900 millivolts; should have been about 500 millivolts. Adjusted it and the idle is much better. Just drove it a few miles before leaving out of town for the past two weeks.

I understand that my parts mish/ mash isn't the best way of doing things, but I did install 33% bigger injectors, (25 lbs vs 19 lbs) since there is about a 33% difference in displacement between a 305 and 400 small block. The computer can still cycle the injectors like it's controlling a 305.

About 99% of my driving is highway cruising to work, with the engine in "closed loop" the ECM should be adjusting the injector pulse to maintain a happy oxygen sensor output.

My version of TPI also uses a Mass Air Flow meter; I'm told these allow greater modifications to engines; giving a little more leeway, before having problems with stock ECM calibrations.

I'll let everyone know how much better it drives when I get some road time...
Thanks,
Mike

383vett

Glad it's working out. Just to let you know, the ecm will increase idle speed if the ac compressor is running. There is no such change for the power steering (the hydraulic pump is always running) or the air pump.

mxw1

Thank you.
Mike